Recently, there has been economic development and the demand for energy has rapidly increased and fossil resources such as natural gas and petroleum are being depleted, thus causing an imbalance between the demand and supply of energy. For this reason, sustainable energy supply systems have been of increasing interest, and the health and environmental risks necessarily associated with carbon dioxide emissions have been recognized. Thus, global efforts to reduce such carbon dioxide emissions have been made. Among a variety of alternative energy sources, biomass can be used as a renewable source of energy and chemical raw materials, particularly as the carbon dioxide which is generated during the conversion of biomass to energy is recycled again when the biomass is produced. Accordingly, biomass is an energy source that emits no carbon dioxide, and it is rich in oxygen compared to other fossil resources, and thus is beneficial for the production of chemical products. Owing to these advantages, many studies on the utilization of biomass have recently been conducted.
Biomass-derived polyols comprise glycerol, butylenes glycol, propylene glycol, ethylene glycol, erythritol etc.
Because biomass-derived polyols are highly useful and can serve as a primary building block in future biorefinery schemes, biomass-derived polyols are a particulary attractive biomass-derived compound. Also, bio-glycerol currently receives a great deal of attention.
Glycerol has generally been a product of the organic chemical industry. However, this source of glycerol is changing with the recent rapid development of the biodiesel industry. Biodiesel became one of the major renewable liquid transportation fuels, and the production of biodiesel involves the production of a large volume of glycerol as a byproduct. The use of this important product stream (about 10 kg of glycerol is produced per 100 kg of biodiesel) provides an important revenue stream. Because of the great usefulness of inexpensive glycerol, it is an attractive raw material which can be chemically converted.
Also, hydrogen is receiving attention as an attractive alternative energy carrier, and hydrogen fuel cells are seen as promising systems producing clean resources and electric power. However, the development of hydrogen production is currently being delayed in terms of efficiency and for environmental reasons, because the hydrogen is mainly produced by the high-temperature steam reforming of non-renewable hydrocarbons.
Because hydrogen production can offer not only economic advantages, but also greater environmental advantages, it is preferable to produce a renewable hydrogen resource such as biomass under mild conditions.
Dumesic et al. reported that hydrogen can be produced by aqueous-phase reforming of biomass-derived oxygenated compounds with a supported metal catalyst at a relatively low temperature (T<538° C.) in a single process. Typical oxygen-containing compounds include methanol, glycol, glycerol, sorbitol, xylose and glucose. Aqueous-phase reforming has an advantage in that it eliminates the need to evaporate water and oxygenated compounds, and thus can reduce the energy required for hydrogen production. Another advantage of the APR (aqueous-phase reaction) process is the production of a negligible amount of carbon monoxide (CO) (an impurity) that is known to act as a poison when H2 is applied in the fuel cell field. This low level of CO results from the low-temperature operation of the APR process, at which a water-gas-shift reaction easily occurs.
The production of hydrogen by aqueous-phase reforming with a supported metal catalyst has the important problem of selectivity. The production of a mixture of CO2 and H2 is thermodynamically unstable compared to the production of methane and higher-molecular-weight alkanes. In addition, the above-described low selectivity for CO and the resulting effective rapid water-gas-shift reaction are particularly important.
Accordingly, a preferred catalyst material should not only minimize the production of CO and alkanes, which can be produced in a series of equilibrium reactions, but also have highly selectivity for hydrogen, and should achieve a high conversion rate of renewable raw materials.
Several types of catalysts were tested regarding the aqueous-phase reforming of renewable oxygen-containing compounds in order to evaluate the effects of the selected transition metals, supports and metal alloys on hydrogen selectivity. It was reported in the literature that a Pt/γAl2O3 [1] and a Sn-modified Raney-Ni catalyst [2] are the most promising catalysts.
A 1-3% Pt/γAl2O3 catalyst showed good results particularly in terms of hydrogen selectivity, conversion rate and stability. Thus, this catalyst is used as a benchmark catalyst for the activity and selectivity of other catalysts.
In most reports on aqueous-phase reforming reactions, fluidized-bed tubular reactors have been used to test activity. Catalyst screening studies were conducted for the purposes of identifying a promising catalyst material which would be used under batch and semi-batch conditions [3].
Various patents and various patent applications were published in the field of aqueous-phase reforming of biomass-derived oxygenated compounds for the production of hydrogen and/or hydrocarbons. The most noteworthy are the patents and patent applications attributed to Cortright and Dumesic, and the relevant patent documents are as follows.
U.S. Pat. No. 6,699,457 (2004) U.S. Pat. No. 6,964,757 (2005) and U.S. Pat. No. 6,964,758 (2005) to Cortright et al. disclose a method of producing hydrogen from oxygen-containing hydrocarbons, including methanol, glycerol, sugar and sugar alcohol, by aqueous-phase reforming in a fixed-bed tubular reactor in the presence of a metal-containing catalyst.
Preferred catalysts described in these patents comprise a metal selected from the group consisting of Group VIII transition metals, alloys thereof, and mixtures thereof. Particularly, the metal is selected from the group consisting of nickel, palladium, platinum, ruthenium, rhodium, iridium, alloys thereof, and mixtures thereof. Platinum, ruthenium or rhodium is the most preferable. The catalyst may comprise an alloy and may be admixed with copper, zinc, germanium, tin or bismuth. Also, according to the above patent documents, the amount of metal added should not exceed about 30 wt % of the VIIIB transition metal catalyst. A support is preferably selected from the group consisting of silica, alumina, zirconia, titania, ceria, carbon, silica-alumina, silica nitride, boron nitride, and mixtures thereof. Silica is preferred [4].
PCT Patent Publication No. WO 2007/075476 of Cortright discloses a bimetallic catalyst for aqueous-phase reforming of oxygenated containing compounds, particularly a combination of a Group VIIIB metal and ruthenium, and most preferably PtRe adhered to a carbon support. Also, it discloses that the addition of an oxide of La or Ce to the catalyst is preferred. In addition, it claims a high feedstock concentration of 20-50 wt % [5].
PCT Patent Publication No. WO 2009/129622 of Monnier et al. discloses a process for aqueous-phase reforming of biomass-derived oxygenated compounds, preferably glycerol, in which a heterogeneous catalyst dispersed in an aqueous phase in a stirred tank reactor (e.g., a continuous stirred tank reactor or a semi-batch stirred reactor). It discloses that platinum and nickel catalysts supported on alumina, silica, activated carbon and zeolite are preferred.
Cortright et al. discloses the oxygenated compounds of methanol, ethylene glycol, glycerol, sorbitol and glucose being converted by an aqueous-phase reforming reaction with 3% Pt/Al2O3. The reaction temperature is 225-265° C., the reaction pressure is 29-56 bar, the concentration of oxygenated compounds in the feedstock solution is 1 wt % [1].
Shabaker et al. discloses aqueous-phase reforming of 10 wt % ethylene glycol solution and shows that platinum supported on TiO2, Al2O3, activated carbon, SiO2, SiO2—Al2O3, ZrO2 or CeO2 and ZnO and platinum supported on TiO2, carbon or Al2O3 are effective [7].
Kunkes et al. reported the conversion of glycerol over carbon-supported Pt and Pt—Re catalysts. The addition of Re increases production of H2, CO, CO2 and light alkanes (mainly methane), and thus increases hydrogen selectivity [8].
Huber Huber et al. reported on the efficiency of a Sn-modified Ni catalyst for aqueous-phase reforming of oxygenated compounds, including ethylene glycol, glycerol and sorbitol (less than 5 wt %) [2].
Haller et al. disclose the use of single-walled carbon nanotubes as a stable support having Pt and Co nanoparticles. It was reported that a Pt—Co bimetallic catalyst shows good activity in aqueous-phase reforming of ethylene glycol [9].
Dong et al. reported Pt-loaded NaY as an active catalyst for aqueous-phase reforming of methanol and ethanol [10].
Souza et al. reported aqueous-phase reforming of ethanol under batch conditions over nickel catalysts prepared from hydrotalcite precursors [11].
Fierro et al. reported aqueous-phase reforming of glycerol over nickel catalysts supported on alumina modified by Mg, Zr, Ce or La. Although Zr, Ce and La increased the initial activity of the catalysts, catalyst inactivation was observed after several hours in every case [12].
Luo et al. developed a cerium catalyst for aqueous-phase reforming, which comprises nickel and cobalt additionally supported on an alumina support. It was observed that cerium suppressed sintering to reduce methane selectivity [13].
Weng et al. reported catalysts for aqueous-phase reforming of glycerol, which comprise Pt, Ni, Co or Cu supported on various supports, including SAPO-11, activated carbon, HUSY, SiO2, Al2O3 and MgO. The platinum catalyst showed the highest stability and activity, and the support also showed increased activity and hydrogen selectivity. The Pt/MgO catalyst showed a great decrease in activity with the passage of time [14].